I had my IIS Express configured as specified on this page http://jasonrshaver.com/?tag=/Client+Certificates
. This allowed me to run my application in SSL for the 11 months from June 2013 all the way up to 9:50 AM EST this morning April 22, 2014. At 9:54 AM EST we ran Visual Studio 2012 Update 4. At 11:00 AM EST I opened my web application and ran a debug. At no point did IE or IIS Express request a client Certificate. The application runs an authentication process which evaluates the server variables and headers to get the Subject value from the client certificate. But there was no Client certificate requested, so there was no Cert_Subject server variable, so the application failed before it could even get to the part I was coding.
All of the settings in both applicationhost.config files are as directed on the web site. This was not happening before VS 2012 Update 4 was run. This started happening immediately after VS 2012 Update 4 was installed. So there is a one to one correlation between these symptoms and the VS 2012 Update 4. What did Update 4 do to my IIS Express. How can I fix it.
I had a similar issue on another machine as note on this thread http://www.experts-exchange.com/Software/Server_Software/Web_Servers/Microsoft_IIS/Q_28372649.html
"Set up of IIS Express to accept Client Certificates working fine until moved to a new machine." However, I went and looked at that machine and sure enough it is using Update 4 of Visual Studio 2012. So it is obviously something in VS 2012 Update 4 that negatively impacted IIS Express.